Beware of latency/lookup delays/RAM usage if you do this. I've done some testing with large lists (around 5,000 ServerAlias directives and up to 16 million Deny from directives) in Apache. 5,000 domains produces a measurable but small delay. When increasing the number, I found the delay started becoming noticeable at around 50,000. With Apache (and probably also with nginx), the list is held in RAM which meant that with the largest set I tried, I could only start one child process within my available RAM.
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LadadadadaJul 6 '12 at 12:07